We have found that the B77 sarcoma virus 60 to 70S genome RNA undergoes a temperature-dependent stabilization process after the virus buds from the cell. In an effort to understand the processes involved in this extracellular stabilization of avian oncornavirus RNA, we will investigate possible changes in the virion ultrastructure, protein composition and endogenous reverse transcriptase activity as a function of the extracellular "age" of the virus. We will also investigate whether the association with virion-associated 4S RNA is involved in the stabilization process. We have found that all the RNA subunits from B77 sarcoma virus contain the blocked and methylated 5' terminal structure m7GpppGm and the methylated nucleoside N6-methyladenosine at internal positions. We plan to determine whether the sites of internal methylation are at unique or at random positions on the RNA molecule and to evaluate the effect of the host cell in determining the sites of methylation. We are also planning to establish at what point in the biosynthesis, processing or maturation of viral RNA that methylation occurs and whether some or all methylation occurs after the virions bud from the infected cell. We will also determine whether methylated bases are present in interesting parts of the viral genome, i.e., those sequences involved in ribosome binding, in linkage of subunits, and in binding of 4S RNAs serving as primers for the reverse transcriptase reaction. BIBLIOGRAPHIC REFERENCES: Stoltzfus, C.M. and Snyder, P.N. 1975. Structure of B77 sarcoma virus RNA:stabilization of RNA after packaging. J. Virol 16, 1161.